Heating of a hermetically sealed packaging pouch filled with a retort food, a frozen food, or the like in a microwave oven increases a pressure inside the pouch due to steam or the like generated from a content under heating. The packaging pouch may suffer rupture, and the content may scatter, so there is a fear that the inside of the microwave oven is made dirty, a human body is burned, or the like.
Thus, before heat-cooking such the packaging pouch in a microwave oven, methods of preventing the rupture of the packaging pouch is employed. That is, the packaging pouch is partly opened in advance or the packaging pouch itself is pierced, to thereby discharge steam or the like generated inside the packaging pouch.
However, such the method involves much time and effort for a general consumer. Further, this method has a disadvantage in that a taste is lost because the steam generated through microwave heating is immediately discharged out of the packaging pouch, and a heating steaming effect of the steam on the content reduces.
Further, to automatically release the increased inner pressure of the packaging pouch at the time of heating in the microwave oven, there have been proposed the following packaging pouches: (1) a packaging pouch in which a weakened seal portion is formed in a part of a peripheral edge heat seal portion of the packaging pouch by interposing a thin film and in which a narrow seal portion with a small heat seal width is provided in a part of the weakened seal portion (see Patent Document 1); (2) a packaging pouch in which a weak adhesive portion is provided in a part of the peripheral edge heat seal portion of the packaging pouch and in which there is formed a non-sealed portion reducing the width of the weak adhesive portion from an outer edge of the weak adhesive portion toward an interior (see Patent Document 2); a packaging pouch in which there are provided in a part of the peripheral edge seal portion of the packaging pouch a non-sealed portion with a locally reduced seal width and an inner swell seal portion corresponding thereto (see Patent Document 3); (4) a packaging pouch in which a heat-shrinkable film forming an opening at the time of heating is sandwiched in a part of an opening portion of the packaging pouch (see Patent Document 4); etc.
[Patent Document 1] JP 10-59433 A
[Patent Document 2] JP 10-95471 A
[Patent Document 3] JP 10-101154 A
[Patent Document 4] JP 10-95470 A
However, these conventional packaging pouches require a complicated process for forming the opening portion, resulting in an increase in cost. Further, the seal strength of the opening portion of the packaging pouch is rather low, so the requisite seal strength (2.3 kg/15 mm-width or more) for a retort food packaging pouch cannot be obtained. Thus, a portion where opening is to be effected or the peripheral edge seal portion suffers rupture at the time of retorting, and there are limitations regarding the content with which the pouch is to be filled. Further, during the transportation or storage of the packaging pouch with the content sealed therein, the pouch will suffer rupture, making it impossible to maintain a sealing property.
The present inventors have already found out and proposed that these problems in the prior art can be eliminated by providing a steam releasing seal portion having a weakened portion, such as a through-hole, a semi-through-hole, or a slit, in a packaging pouch hermetically sealed by heat-sealing with a plastic film (see Patent Document 5).
Further, in such the packaging pouch provided with a steam releasing seal portion having a weakened portion, to prevent separation of an interface not contributing to the sealing of a steam releasing seal portion composed of a laminated film due to some impact during the process of forming a weakened portion, such as a through-hole, a semi-through-hole, or a slit, in the steam releasing seal portion, the present inventors have proposed a construction in which, as shown in FIG. 15, a steam releasing seal portion 6 is formed by an outer seal portion 3, a cushioning portion 4 adjacent to the outer seal portion 3, and a weakened portion 5 provided in the cushioning portion 4 (see Patent Document 6).
[Patent Document 5] JP 2002-249176 A
[Patent Document 6] JP 2003-192042 A
The packaging pouches as disclosed in Patent Documents 5 and 6 each of which is provided with a steam releasing seal portion having a weakened portion help to eliminate the problems in the prior art as disclosed in Patent Documents 1 through 4, making it possible to attain the desired object. It should be noted, however, that, when these packaging pouches are filled with solid objects of a small size, such as boiled rice, rice porridge, or curry roux, and hermetically sealed before being heat-cooked in a microwave oven, the pressure in the pouch increases due to the steam, etc. generated from the content as a result of the heating. When an opening is generated in the steam releasing seal portion through separation thereof, clogging may occur in the steam releasing seal portion by a small solid object such as a rice grain, thereby interfering with the automatic opening of the steam releasing seal portion.